1. Field of the Invention
The present invention relates generally to protection switching between active and standby systems, and more specifically to protection switching of a communication network where transmission lines are duplicated for backup purposes.
2. Description of the Related Art
Japanese Laid-Open Patent Specification 2-260729 discloses a duplicated network of active and standby transmission systems between local and remote control units such as time-division multiplexers. Each of the active and standby transmission systems includes a local line termination unit and a remote line termination unit interconnected by high-seed digital dedicated lines. Each of the local and remote control units transmits signals in the coded mark inversion (CMI) format, As a means for establishing frame synchronization, the rule of violation is utilized. If a failure occurs in the active system, the local line termination unit loses track of a transmitted frame and sends a unit-not-ready signal to the local control unit, recognizing that a failure has occurred. The local control unit then stops sending the frame synchronization signal to the remote line termination unit. The latter responds to the lost of a transmitted frame by sending a unit-not-Receive signal to the associated control unit, signalling the occurrence of a failure in the active system. Protection switching from the failed active system to the standby system is then performed by the local and remote control units.
However, in transmission systems where the rule of violation is not used as a means of establishing frame synchronization or where the type of units that produce a unit-not-ready signal in the event of a failure is not used, it is impossible to signal the occurrence of a system failure to the remote communication unit. In addition, since the loss of frame synchronization does not necessarily indicate the actual occurrence of a failure, protection switching may take place when the system loses synchronization due to a cause other than a system failure, resulting in a difficulty to distinguish between a system failure and a loss of synchronization.
It is therefore an object of the present invention to provide a communication network capable of protection switching of duplicated transmission systems of any type of line codes while eliminating the need to provide distinction between actual failures and simulated failures.
According to a first aspect of the present invention, there is provided a network node connected to a remote network node by an active transmission system and a standby transmission system, comprising active receive circuitry for receiving communication signals from the active transmission system and detecting an operating state of the active transmission system, active transmit circuitry for transmitting communication signals over the active transmission system to the remote network node, standby receive circuitry for receiving, from the standby transmission system, a status signal indicating an operating state of the active transmission system detected by the remote network node from the transmitted communication signals, standby transmit circuitry for transmitting a status signal indicating the operating state detected by the active receive circuitry over the standby transmission system to the remote network node, and control circuitry responsive to an operating state detected by the active receive circuitry and an operating state indicated by the status signal of the standby receive circuitry for performing a protection switching between the active and standby transmission systems.
According to a second aspect, the present invention provides a protection switching method for a local network node and a remote network node interconnected by an active transmission system and a standby transmission system, comprising the steps of (a) receiving, at the local network node, communication signals from the remote network node over the active transmission system, and detecting an operating state of the active transmission system, (b) transmitting, from the local network node, a first status signal indicating the detected operating state over the standby transmission system to the remote network node, (c) receiving, at the remote network node, communication signals from the local network node over the active transmission system, and detecting an operating state of the active transmission system, (d) transmitting, from the remote network node, a second status signal indicating the detected operating state over the standby transmission system to the local network node, (e) receiving the first status signal of the local network node at the remote network node over the standby transmission system, (f) receiving the second status signal of the remote network node at the local network node over the standby transmission system, and (g) performing a protection switching at the local and remote network nodes between the active and standby transmission systems in accordance with the first and second status signals.